The invention concerns a process for preventing oscillating shifting of the gears of a motor vehicle automatic transmission which is provided with an electronic transmission control device.
In the context of certain road resistances (such as driving uphill with a trailer), oscillating gearshifts occur as a result of the fact that in a lower gear when driving under a full load the acceleration is always positive, while in the next higher gear the acceleration is always negative. After a shift which must be carried out in order to prevent the engine from exceeding its rated speed of rotation, the vehicle speed drops off again despite the full load until the hysteresis for downshifting has been exceeded.
From EP 0 784 170 B1 a process is known for controlling the upshifting process of the motor vehicle automatic transmission. It controls the upshifting or downshifting as a function of the throttle valve opening angle and the vehicle velocity by making use of stored upshifting characteristic lines or downshifting characteristic lines. After an upshifting characteristic line is exceeded by the actual operating point of the transmission control device before the initiation of an upshifting process, it checks whether a defined minimal acceleration is possible in the higher gear according to the momentary vehicle resistance calculated from the current values of the throttle valve opening angle, the engine speed, the engine torque, the vehicle speed, the gear ratio and the converter amplification degree without falling below the downshifting characteristic line. In the case of a negative test result, the upshifting process is suppressed, and in the case of a positive test result, the upshifting process is executed. In this case the defined minimal acceleration is stored in a control device as a magnitude coordinated with the vehicle velocity. Other procedures and devices for controlling the gearshifting process in a motor vehicle automatic transmission are known from DE 196 00 914 A1, DE 41 11 645 A1 and DE 38 30 938 C2.
It is an objective of the present invention to devise a different, especially simple process for preventing oscillating shifting in the automatic transmissions of motor vehicles.